Development within wireless technology has been, and still is, on rampage. The use of wireless communications networks, sometimes also referred to as cellular communications networks, cellular radio system or cellular networks, continues to grow rapidly. New wireless technologies and standards are constantly emerging. Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM) and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access. The 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) has undertaken to evolve further the UTRAN and GSM based radio access network technologies, and 3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic.
A wireless device, which may also be referred to as e.g. a user equipment (UE) terminal, mobile terminal, wireless terminal and mobile station, may be enabled to communicate wirelessly in any of such wireless communications networks. A wireless communications network may cover a geographical area which is divided into cell areas, wherein each cell area is served by a base station. A cell is the geographical area where radio coverage is provided by the base station at a base station site. Each base station may serve one or several cells, and furthermore, each base station may support one or several communication technologies and be directly connected to one or more core networks. Depending on the technology and terminology used, a base station may be referred to as e.g. a Radio Base Station (RBS), Base Transceiver Station (BTS), B node, NodeB, Evolved Node B (eNodeB), or eNB, and the term “base station” is used in this description to denote any of these. The base stations may communicate over the air interface operating on radio frequencies with the wireless devices within range of the base stations.
A wireless device may be subjected to handover from one cell to another. One reason may for instance be that more efficient utilization of capacity is sought after, and another that the wireless device is moving away from an area covered by one cell—the source cell—and is entering an area covered by another cell—e.g. a target cell—which therefore provides better radio conditions for the wireless device. For practical realization of handover, each cell may be assigned a list of potential target cells, which can be used for handover. These potential target cells may be referred to as neighbour cells and the list of potential target cells referred to as a neighbour list. Creating such a list for a given cell is not trivial, and implementations come in a wide variety. For instance, different algorithms may utilize input data from field measurements or computer predictions of radio wave propagation in the areas covered by the cells.
Manually provisioning and managing neighbour cells in wireless networks may be challenging, and it becomes more difficult as new technologies are being rolled out while 2G/3G cells still remain. In the case of 3GPP LTE, for instance, in addition of defining intra LTE neighbour relations (NRs) for eNBs, operators need to provision neighbouring 2G, 3G, CDMA2000 cells as well. Accordingly, a purpose of the Automatic Neighbour Relation (ANR) functionality of 3GPP LTE is to relieve the operator from the burden of manually managing NRs. The ANR function may reside in an eNB and support a neighbour list by means of the conceptual Neighbour Relation Table (NRT). The ANR function may allow e.g. Operation & Maintenance (O&M) to be informed about changes, define attributes, and manage the NRT, such that O&M can add and delete NRs. Furthermore, the implementation specific Neighbour Detection Function may find new neighbours and add them to the NRT, while the Neighbour Removal Function may remove outdated NRs.
In order to manage NRs in an automatic manner, a source base station of a source cell may, as a part of a normal call procedure, instruct a wireless device in the source cell to perform measurements on neighbour cells. In response to such an instruction, the wireless device may send a measurement report regarding a specific neighbour cell, which for instance is triggered as a consequence of a strong Reference Signal Received Power (RSRP). The report may contain the specific cell's Physical Cell Identifier (PCI), and upon reception thereof, the source base station may instruct the wireless device to read, and report back, the Eutran Cell Global Identifier (ECGI), the Tracking Area Code (TAC), and all available Public Land Mobile Network (PLMN) ID(s) of that specific cell. Next, the source base station may decide to add a NR for the cell to the NRT, whereby that neighbour cell may be applicable for handover of a wireless device from the source cell.
Handover attempts to a neighbour cell may, however, fail. A handover procedure may fail whenever there is a failure at any stage in the handover process, and the failure may be related to hardware as well as software issues. On one hand, failure may occur when a source base station sends a handover request message to a target base station, and the target base station rejects the request by responding with a handover preparation failure message. This could be due to many reasons, for instance that there are no radio resources available in the target cell or due to a hardware failure. Reasons of failure are, in the case of LTE, listed in 3GPP 36.423 E-UTRAN X2 Application Protocol (sections 8.2.1.3 Unsuccessful Operation and 8.2.1.4 Abnormal Conditions).
On the other hand, however, based on field experience, the handover procedure may also fail at other stages, not necessarily listed. This might be due to hardware or software issues at the target base station, which may cause failure of all incoming handovers from the source cell to that target cell. The target cell may, for instance, be broken or malfunctioning already at the time for the cell relation setup. That is, the target cell that broadcasts, seemingly working from a wireless device perspective, may for instance be sleeping, i.e. the cell's base station is not operating. While such a fault as a sleeping cell remains, one should be careful not to handover a wireless device in that direction. Although a sleeping cell can manifest itself in many ways, the wireless device to be handed over may be destined to face a drop. Thus, one could argue that the relation to a sleeping cell should be removed, and that the source cell subsequently should not consider the target cell a valid neighbour. However, the erroneous condition of the sleeping cell is likely temporary, and one can thus just as well argue that the cell relation should be kept, i.e. that the target cell should remain a valid neighbour. It may still be of value to keep the relation, for instance to support Inter Cell Interference Coordination (ICIC), even though the target cell is not responding to handover requests from the source cell. Thereby, there is, on one side, a risk of the wireless device facing a drop, and on the other, that numerous handover attempts merely results in handover failures. Which approach to take when faced with hardware and/or software issues giving rise to such handover failure behavior, is hence not necessarily obvious.